1. Field of the Invention
The present invention relates to a hollow cylindrical thermal shield for a tubular cryogenically cooled superconducting magnet, and particularly to such a thermal shield which is useful in MRI (Magnetic Resonance Imaging) systems due to reduction in gradient coil induced heating (GCIH) of cryogenically cooled apparatus.
2. Description of the Prior Art
Superconducting magnets for use in MRI systems are commonly cylindrical in shape, and the present invention is directed to such magnets. In an MRI system, a gradient coil assembly provides pulsed magnetic fields to provide the required spatial encoding of the imaging volume. Such time-variant magnetic fields will induce heating into conductive materials in the vicinity.
FIG. 1 illustrates a typical arrangement of an MRI magnet system. Coils 10 are wound onto a former (not shown) which is placed within a cryogen vessel 12. The cryogen vessel is partially filled with a liquid cryogen 15 such as helium to provide the required cooling. A thermal radiation shield 16 surrounds the cryogen vessel to shield it from radiated heat. The cryogen vessel and the thermal shield are cooled by a cryogenic refrigerator 17. The coils, former, cryogen vessel and thermal radiation shield are surrounded by an outer vacuum chamber (OVC) 14. The volume between the outer vacuum chamber 14 and the cryogen vessel 12 is evacuated. Solid thermal insulation 18, such as aluminized polyester film, is preferably placed in the space between the outer vacuum chamber 14 and the thermal radiation shield 16. Numerous other components, such as mechanical support structures, are provided in a practical MRI magnet system, but are not illustrated in the drawing for the sake of clarity. In designing an MRI system, great effort is placed on reducing thermal influx to the cryogen vessel; on maximizing the diameter of the bore of the outer vacuum chamber; and on reducing its length.
A cylindrical gradient coil assembly is typically placed within the bore of the outer vacuum chamber.
The cryogen vessel, thermal radiation shield and outer vacuum container are each typically hollow cylindrical enclosures, each composed of an inner tube, an outer tube and two annular end pieces joining the inner tube and the outer tube.
The inner tube of the thermal radiation shield is typically of a highly electrically and thermally conductive material, such as pure aluminum, and is about 6 mm thick. Such material is effective at shielding the cryogen vessel from high-frequency (>100 Hz) varying magnetic fields from the gradient coil assembly. Relatively large eddy currents may be induced in the inner tube of the thermal radiation shield due to the pulsing of a magnetic field by the gradient coils. Such eddy currents cause heating of the thermal radiation shield.
However, secondary and tertiary eddy currents remain a problem. Although the cryogen vessel is not subjected to the high-frequency varying magnetic fields of the gradient coils, the magnetic pulsing of the gradient coils causes mechanical vibration of the OVC and the thermal radiation shields. These vibrations, within the magnetic field of the coils, cause induced eddy currents in the material of the OVC and the thermal radiation shields. These induced eddy currents in turn cause heating; and the magnetic fields generated by the induced eddy currents induce further eddy currents, and cause heating, in the cryogenically cooled components such as coils 10 and cryogen vessel 12. All of such heating is collectively known as gradient coil induced heating (GCIH).
The heating is particularly pronounced in cases where the pulsing of the gradient coils is at a frequency near the resonant frequencies of the inner tube of the OVC and the inner tube of the thermal radiation shield. It is believed that the proximity of the resonant frequencies is a feature of nested tubes of similar dimensions, even where the tubes are of differing materials.
In magnet systems such as illustrated in FIG. 1, the coils 10 themselves are cooled by liquid cryogen 15 and will not be heated by the GCIH. However, an increased boil-off of cryogen will occur due to GCIH of the cryogen vessel and the coils and radiant heating caused by GCIH of the thermal radiation shield.
Recent developments have led to magnets described as “low cryogen inventory” or even “dry” magnets. In such designs, little or no liquid cryogen is provided to cool the magnets. In “low cryogen inventory” magnets, a relatively small volume of cryogen circulates in thermal contact with the magnet coils, and is cooled by a cryogenic refrigerator as it circulates. In a “dry” magnet, no cryogen is provided, but a cryogenic refrigerator is thermally linked to the magnet through a thermally conductive link such as a copper or aluminum braid or laminate.
In “low cryogen inventory” or “dry” magnets, there is not a large volume of cryogen to absorb heating of the cryogen vessel or the shield due to GCIH. As a result, there is a risk that the coils 10 will heat, and quench, even in response to a relatively small amount of heating. It is therefore particularly important to minimize GCIH in “low cryogen inventory” or “dry” magnets. This may be addressed by intercepting heat generated by GCIH, either in the gradient coils, at the OVC inner tube, or at the thermal shield. The present invention is particularly directed to intercepting the majority of heat resulting from GCIH at the thermal radiation shield.
Some attempts have already been made to address this problem. In some arrangement (e.g. U.S. Pat. No. 7,514,928), the cryogen vessel has been coated or lined with copper. This does not prevent or reduce the magnitude of eddy currents in the cryogen vessel, but reduces the resultant heating due to the reduced electrical resistance of the cryogen vessel. This approach has been found to have limited success, as the reduced resistance of the cryogen vessel has been found to lead to increased eddy currents.
The mechanical vibration of the inner tube of the thermal shield has been addressed (e.g. U.S. Pat. No. 7,535,225) by bonding patches of a high modulus material, such as carbon-fiber reinforced plastic CFRP, onto the shield's inner tube. Such an approach has been found effective to change the resonant frequency of the shield's inner tube only if a significant radial thickness of stiffening material is used. This results in an increase in the diameter of the coils, and a great increase in wire cost, in order to keep the bore of the OVC at the required diameter.